Data Transmission

ABSTRACT

Audio-visual data is provided to a network-connected terminal. First audio-visual data containing first information encoded at a first data rate and second audio-visual data containing first information encoded at a second data rate that is more than the first data rate are received. The first audio-visual data is provided to the network-connected terminal. A condition is identified to the effect that the amount of data being sent to the terminal is less than the available bandwidth. The difference between the data rates of the first and second audio-visual data is calculated, additional data packets are sent to the terminal to determine whether the available bandwidth is large enough to provide the first audio-visual data to the terminal; and if it is determined that the available bandwidth is large enough, provision of the first audio-visual data to the terminal is stopped and the second audio-visual data is provided instead.

TECHNICAL FIELD

The present invention relates to a method of providing data over anetwork.

BACKGROUND OF THE INVENTION

It is possible to wirelessly provide audio-visual data such astelevision channels, often via the Internet, to mobile devices such asmobile telephones. However, because available bandwidth frequentlyfluctuates it is generally necessary to send data representing a lowquality of picture and sound in order to avoid high packet loss and adegraded user experience.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided amethod of providing audio-visual data over a network, comprising thesteps of receiving first audio-visual data containing first informationencoded at a first data rate, and second audio-visual data containingfirst information encoded at a second data rate that is higher than thefirst data rate; providing the first audio-visual data to anetwork-connected terminal; identifying a condition to the effect thatthe amount of data being sent to the terminal is less than the availablebandwidth; calculating the difference between the data rates of thefirst and second audio-visual data; sending additional data packets tothe terminal to determine whether the available bandwidth is largeenough to provide the first audio-visual data to the terminal; and if itis determined that the available bandwidth is large enough, stoppingprovision of the first audio-visual data to the terminal and providingthe second audio-visual data to the terminal.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a networked environment;

FIG. 2 shows a PDA shown in FIG. 1;

FIG. 3 shows illustrates the contents of the memory of the PDA shown inFIG. 2;

FIG. 4 shows steps carried out by a media player on the PDA shown inFIG. 2;

FIG. 5 details steps carried out in FIG. 4 to negotiate a communicationslink;

FIG. 6 illustrates the relationship between display time and server timein data sent over the communications link negotiated in FIG. 5;

FIG. 7 details steps carried out in FIG. 4 to receive and display data;

FIG. 8 illustrates the PDA shown in FIG. 2 displaying data;

FIG. 9 shows steps carried out by a content application on the PDA shownin FIG. 2;

FIG. 10 details steps carried out in FIG. 9 to obtain and display data;

FIG. 11 illustrates the PDA shown in FIG. 2 displaying a channel guide;

FIG. 12 shows a content server shown in FIG. 1;

FIG. 13 shows the contents of the memory of the content server shown inFIG. 12;

FIG. 14 details steps carried out by a media controller on the contentserver shown in FIG. 12;

FIG. 15 shows a media server shown in FIG. 1;

FIG. 16 illustrates how the media server shown in FIG. 15 receives andtransmits data;

FIG. 17 shows the contents of the memory of the media server shown inFIG. 15;

FIG. 18 details steps carried out by a channel manager on the mediaserver shown in FIG. 15;

FIG. 19 details steps carried out in FIG. 18 to define a user channel;

FIG. 20 details steps carried out in FIG. 19 to set up the user channel;

FIG. 21 details steps carried out in FIG. 18 to alter a user channel;

FIG. 22 illustrates the effect of altering the user channel during FIG.21;

FIG. 23 details steps carried out in FIG. 21 to change display timeoffsets;

FIG. 23 a details steps carried out in FIG. 23 to augment the displaytime offsets;

FIG. 24 illustrates the effect of further altering the user channelduring FIG. 21;

FIG. 25 illustrates the transmission and receipt of data between themedia server shown in FIG. 15 and the PDA shown in FIG. 2;

FIG. 26 details steps carried out by a Quality of Service manager on themedia server shown in FIG. 15;

FIG. 27 details steps carried out in FIG. 26 to change the quality of atelevision channel;

FIG. 28 details steps carried out in FIG. 27to step up the quality;

FIG. 29 details steps carried out in FIG. 27 to step down the quality;

FIG. 30 details a probing procedure; and

FIG. 31 illustrates steps up and steps down in response to availablebandwidth.

DESCRIPTION OF THE BEST MODE FOR CARRYING OUT THE INVENTION FIG. 1

FIG. 1 illustrates a networked environment in which the invention may beused. Terminals 101, 102, 103, 104, 105, 106 and 107 receive data viathe Internet 108. The terminals are typically mobile telephones orPersonal Digital Assistants (PDAs) but could be any computing devicecapable of connecting to the Internet, including a home or officecomputer.

The data is provided over a variety of networks, including in thisexample radio networks such as mobile telephony networks or wirelessnetworks. A Third Generation (3G) mobile telephony network, connected tothe Internet 108, includes a gateway 109 which provides connectivity toa network of base stations. Mobile telephones 102 and 103 are eachconnected to one of these base stations. A General Packet Radio Service(GPRS) gateway 110 is connected to the Internet 108 and providesconnection to a network of GPRS base stations. PDAs 104 and 105 are eachconnected to one of these stations. A GSM gateway 111 is connected tothe Internet 108, providing connectivity for mobile telephone 101.Internet Service Provider (ISP) 112 is connected to the Internet 108 andprovides internet access for PC 106 and a Wireless Network or WirelessFidelity (WiFi) gateway 113. PDA 107 has a link to gateway 113. Thusthere is a number of ways in which a terminal may link to the Internet108 in order to receive data.

The data received by terminals 101 to 107 is provided by content server114 and media server 115. Content server 114 provides many kinds of datarequired by users of terminals, for example news and sports, financialdata, maps and telephone directories, television and cinema listings,and so on. Some of this data may be available to subscribers and somemay be free. Users may make transactions with content server 114 such aspurchasing stocks or placing bets, changing their subscription levels orpersonal details, and so on.

Media server 115 provides streamed audio-visual data such as televisionchannels, media-on-demand or downloadable music videos. Streamedtelevision channels are provided to media server 115 by Real TimeStreaming Protocol (RTSP) servers 116, 117 and 118.

FIG. 2

FIG. 2 details PDA 104. As described above, this is an example of aterminal that could be used in a system embodying the invention. Itincludes a CPU 201 with a clock speed of 400 megahertz (MHz) with memory202 being provided by 64 megabytes (MB) of RAM. 256 MB of non-volatileFLASH memory 203 is provided for program and data storage. Liquidcrystal display 204 is used to display information to the user.Input/output 205 processes the input of the keys and buttons 513 whileaudio input/output 206 provides a microphone and speaker interface foruse with the telephone facility. Universal Serial Bus (USB) input/output207 is used to connect PDA 511 to another computer, or to the Internet110 via a wired connection. GPRS/WiFi connection 208 and GSM connection209 enable PDA 511 to connect to wireless networks, while Ethernet card210 enables PDA 511 to connect to a wired network, for example via adocking station on a computer.

FIG. 3

FIG. 3 illustrates the contents of memory 202 of PDA 104. An operatingsystem 301 provides overall functionality for the device and contentapplication 302 communicates with content server 114 to obtain anddisplay data required by the user and to make transactions. Music player303 is a plug-in that displays audio-visual data. This data may bestored on PDA 104 or provided by media server 115.

Memory 202 also contains data 303, which includes such data asundisplayed contents data or media data, packets to be sent or that havenot been acknowledged, and data required by operating system 301 andcontent application 302.

FIG. 4

Media player 303 is a “black box” plug-in with a simple API. It willgenerally only play data that is sent using the Real Time Protocol(RTP), although it may also comprehend a proprietary protocol, and willaccept a very limited number of commands. A media player application on,for example, a desktop computer would be much more sophisticated butsince the invention described herein is designed to function on any kindof terminal it must interact with media players having the least amountof functionality, such as that described herein.

Audio-visual data is typically sent from an RTSP (Real Time StreamingProtocol) server to an RTSP client over UDP/IP using a communicationslink that is negotiated using RTSP over a TCP/IP link. It comprises datapackets, sent in two RTP streams, one for video data and one for audiodata, and two RTCP (Real Time Control Protocol) streams which containcontrol packets. A media player is generally configured to act as anRTSP client.

Steps performed by media player 303 are described in FIG. 4. At step 401the player is initialized by operating system 301 or content application302. At step 402 a communications link is set up. This may be to datastored locally on flash memory 203, as is likely if the player has beeninitialized by operating system 301, or to a remote location as is thecase if it has been initialized by content applications 302. For thepurposes of this description, the communications link is with mediaserver 115 via Internet 108.

At step 403 audio-visual data is received over the communications linkand displayed on LCD 204. Additionally, report packets are generated andsent over the communications link. These report packets confirm that thelink is open and also contain information such as the number of lostpackets.

At step 404 a STOP request is received, usually as a result of the userstopping playback on the device, and thus at step 405 a TEARDOWN requestis sent over the communications link. This closes the communicationslink and at step 406 a reply is received confirming this. At step 407 aquestion is asked as to whether the user restarts playback and if thisquestion is answered in the affirmative control is returned to step 402and a new communications link is opened. If it is answered in thenegative then the plug-in is terminated at step 408.

FIG. 5

FIG. 5 details step 402 at which a communications link is defined by aseries of Real Time Streaming Protocol (RTSP) messages sent over TCP. Atstep 501 a universal resource locator (URL) is received. This URL muststart with “rtsp://” and be followed by an address of a server, a portnumber, and an identification of what is to be played, such as a filename. The URL is usually passed to the player in the form of a textstring. At step 502 the player identifies the server address and portfrom the URL and at step 503 it makes a TCP connection with theidentified port on the identified server. Once this TCP connection ismade a DESCRIBE request is transmitted over it. This requests the serverto describe the data that is going to be sent and thus at step 505 areply is received and the required information is extracted at step 506.

The information received includes an indication as to how many streamsthere are within the data. If the data is audio only then the indicationwill be that there is one stream, while for audio-visual data it will betwo. Currently there is no other type of data that can be streamed butprovision is made within the protocol for as many streams as arenecessary. Thus at step 507 a SETUP request is transmitted for the firstidentified stream. This request includes the number of the port on thedevice to which the stream of data packets is to be delivered, and alsothe port to which the stream of control packets associated with thestream of data packets is to be delivered. Typically, these port numberswill be consecutive. At step 508 a reply is received from the serverindicating the ports that it is using and at step 509 a PLAY request issent to start the sending of packets.

At step 510 the question is asked as to whether there is another streamto be set up. If this question is answered in the affirmative thencontrol is returned to step 507 and a SETUP request is transmittedagain. However, if all streams have been set up then the question isanswered in the negative and step 402 is completed.

Once this communications link is established then data is sent usingRTP, typically over UDP since acknowledgements are not generallyrequired but over TCP if UDP is not possible, to the two identifiedports. Additionally, control packets are sent using the Real TimeControl Protocol (RTCP) to the two additional ports. The original TCPconnection may then be broken.

Thus it can be seen that for a typical communications link along which astream of audio data and a stream of video data are to be sent, a totalof six TCP requests must be made before data can be transmitted, and anynumber of these requests may need to be retransmitted if packets arelost. This can take a long time, particularly on a wireless link, andthus it may take between ten and twenty seconds to set up acommunications link before data can be streamed to a terminal. This maybe acceptable when the user wishes to view a single item, such as in avideo conferencing environment. However, in the environment shown inFIG. 1 the user of a terminal may be watching television channels on theterminal. Such a user typically wishes to change channel fairlyfrequently, for example when “channel surfing” to find a suitableprogramme to watch, when flicking between channels to check whether aprogramme has begun, when changing to another channel during acommercial break, and so on. Using the current system, every time theuser wished to change channel a new communications link would have to benegotiated. Clearly, a user will not tolerate a twenty-second delayevery time he wishes to change channel.

FIG. 6

Once the communications link is set up, data packets are streamed to PDA104 using RTP. For each RTP stream, there is an RTCP stream of controlpackets. Thus the data comprises two sets of streams, each setcomprising a stream of data packets and a stream of control packets.

The control packets contain information necessary to allow media player303 to display the audio-visual data correctly. In particular, theyrelate a display time to an server time. Each stream of RTP packets isconsidered to have a display time as exemplified in the graph shown inFIG. 6. This display time is linearly related to the server time and canbe described using a gradient and an offset. The RTSP server thatgenerates the RTP data packets also generates a random gradient andoffset for each stream, (a preset gradient and random offset may be usedinstead). These are used to generate a display time for each RTP packetin that stream. The player receiving that packet must convert thedisplay time into an server time in order to decide when the dataincluded in that packet should be displayed.

As shown in FIG. 6, a video stream 601 includes a plurality of videodata packets such as packets 602, 603, and 604. An audio stream 605includes a plurality of audio data packets such as packets 606, 607 and608. These are more infrequent than the video data packets because audiodata is typically smaller. Streams 601 and 605 have different gradientsand different offsets. Thus at time t, the display time for video stream601 is shown by line 609 while the display time for audio stream 605 isshown by line 610. Thus two data packets that should be displayed atsimilar times may have very different display times.

Since the audio stream and video stream have different display times,additional information is required to display the audio and video datasynchronously. Each control packet for each stream contains the servertime at which the control packet was generated and the correspondingdisplay time. Different players use this information in different ways,but typically the gradient is defined during setup, and an extrapolationis made from the last few received control packets to define an adderwhich when combined with the defined gradient gives a time definition.This is used to determine what the server time is of a received RTPpacket. Alternatively, a client may also be able to determine thegradient using extrapolation.

FIG. 7

FIG. 7 details step 404 at which data is received and displayed by mediaplayer 303. At step 701 a packet is received and at step 702 a questionis asked as to whether it is a control packet, which can be determinedby which port a packet arrives on. If this question is answered in theaffirmative then at step 703 the time definition for the relevant streamis updated. If the question is answered in the negative then the packetis a data packet and at step 704 the display time for the data packet iscalculated from the current time definition. At step 705 the packet isdisplayed at the correct time. It may be that data packets receivedlater than the current data packet should be displayed earlier, or viceversa. This is because packets may be routed differently and take ashorter or longer time to arrive. Also, packets may be lost and thusthere may be times at which no data is displayed at all. However, thisis not likely to be very noticeable to the user as long as the packetloss is not too high.

At step 706 a question is asked as to whether a report packet should begenerated. These are sent every few seconds, and thus if the question isanswered in the affirmative then a report packet is generated andtransmitted at step 707. Amongst other information, this report packetcontains a loss fraction. Because all data packets are numberedsequentially it is possible for the player to know how many packets havenot been received and this information is included in the report packet.

At this stage, and following step 703, a question is asked as to whetheranother packet has been received. If this question is answered in theaffirmative then control is returned to step 702 and the packet isprocessed. If it is answered in the negative then step 404 is completed.

FIG. 8

An illustration of PDA 104 is shown in FIG. 8. Audio-visual data isdisplayed on LCD 204 and the user can control the device using buttons801. In the illustration, the user is viewing a news channel.

According to prior art systems, if the user wished to change channel theplayer 303 would terminate the communications link and negotiate a newone as described with respect to FIG. 5, which could take as long astwenty seconds. However, according to the invention described herein,the user may change channel much more quickly.

FIG. 9

FIG. 9 details content application 302. This is an application loaded onPDA 104 that communicates with content server 114 via internet 108 inorder to supply content to the user as required and also to facilitatetransactions made by the user. Content application 302 communicates withmedia player 303 using the limited range of commands that the playerwill accept, typically only PLAY, STOP or PAUSE. It will also accept atext string containing a URL as described with reference to FIG. 5. Atstep 901 the content application 302 initializes when the PDA 104 isswitched on. At step 902 the application obtains and displays content asrequired by the user. At step 903 a question is asked as to whether theuser wishes to view media, indicated by the user making certainkeypresses using keys 801 based on options displayed on LCD 204, and ifthis question is answered in the affirmative then at step 904audio-visual data is obtained and displayed. At step 905 a question isasked as to whether the user is closing the application. This usuallyoccurs when the device is switched off, but the application can beclosed at any time. If this question is answered in the negative thencontrol is returned to step 902 and if it is answered in the negativethen the application closes at step 906.

FIG. 10

FIG. 10 details step 904 during which audio-visual data is obtained anddisplayed. At step 1001 a channel guide is obtained and displayed on LCD204. This guide indicates to the user which programs are being shown onavailable channels and at what time. Channels that are not available tothe user may also be shown, for example to encourage the user toincrease his subscription level. At step 1092 the user selects a channelto view and at step 1003 a CHANNEL PLAY request is sent to contentserver 114. The content server generates a URL and returns it to thedevice over internet 108 and thus at step 1004 a question is asked as towhether a URL has been received. If this question is answered in thenegative then either no message or an error message has been receivedfrom content server 114, and so the message “CHANNEL NOT PERMITTED” isdisplayed at step 1005 before control is returned to step 1002 for a newchannel selection. However, if it is answered in the affirmative then atstep 1006 media player 1003 is initialized and at step 1007 the receivedURL is passed to the player in the form of a string of text. The URLpoints the player to media server 115, and upon receipt the playercarries out the steps shown in FIG. 5 to set up a communications link tostart streaming and displaying audio-visual data.

At step 1008 a question is asked as to whether the user wishes to changethe channel, indicated by the user making certain keypresses using keys801 based on options displayed on LCD 204. If this question is answeredin the affirmative then at step 1009 a CHANNEL CHANGE request is sent tocontent server 114. As will be described further with respect to FIG.14, upon receipt of this request content server 114 requests mediaserver 115 to change the data sent via the established communicationslink. Thus the player receives different data, ie a new channel, withouthaving to terminate and renegotiate the communications link. The channelchange thus appears virtually seamless to the user.

Thus at step 1010 a question is asked as to whether the message “NOTPERMITTED” has been received, and if this question is answered in theaffirmative then the message “CHANNEL NOT PERMITTED” is displayed to theuser. Otherwise, the channel change has been carried out and in bothcases then control is returned to step 1008. Eventually the questionasked at step 1008 is answered in the negative and at step 1012 the userstops play in the media player. The player is closed at step 1013 andstep 904 is complete.

FIG. 11

PDA 104 is again illustrated in FIG. 11. The user has indicated that hewishes to change channel and so content application 302 overlays achannel guide 1101 on the display 204. The user may select a channel towatch using buttons 801, following which the content application 302contacts the media server 114 to change channel. Media server 115 sendsthe packets of a different channel down the existing communications linkand thus the channel change is transparent to the media player 303.However, because each data stream of the new channel will have adifferent display time from the current streams, they cannot be simplyswitched over. The player will continue to apply its existing timedefinitions to the new packets, thus leading to a calculation of servertime that could be wrong by seconds, minutes, days or even years.Further, the new channel's data streams will have different packetsequence numbers from the original streams.

FIG. 12

FIG. 12 shows content server 114. It comprises two parallel centralprocessing units (CPUs) 1201 and 1202 having a clock frequency of 3 GHz,a main memory 1203 comprising 4 GB of dynamic RAM and local storage 1204provided by a 20 Gb-disk array. A CD-ROM disk drive 1205 allowsinstructions to be loaded onto local storage 1204 from a CD-ROM 1206. Afirst Gigabit Ethernet card 1207 facilitates intranet connection, andcan also be used for installation of instructions. A second GigabitEthernet card 1208 provides a connection to Internet 108.

FIG. 13

The contents of main memory 1203 are illustrated in FIG. 12. Operatingsystem 1301 provides operating system instructions for common systemtasks and device abstraction. In this example a Windows® Serveroperating system is used, but another system providing similarfunctionality could be used. Content serving applications 1302 includeinstructions for delivering content to terminals, updating personaldetails, making transactions, and so on. Media controller 1303 receivesrequests from terminals for the playing of audio-visual data, includingchannel change requests, and communicates with media server 115. Data1304 includes session data for each user, buffered messages, and otherdata used by operating system 1301, content serving application 1302 andmedia controller 1303.

FIG. 14

FIG. 14 details steps carried out by media controller 303 to serverequests from terminals such as PDA 104. At step 1401 it starts,typically during the starting of content server 114, and at step 1402 arequest to view a channel is received from a content application on aterminal, such as that sent by content application 302 at step 1003. Atstep 1403 a question is asked as to whether the user is permitted toview the requested channel. This is done by loading user data from harddrive 1204 into main memory 1203 and checking the user permissions. Ifthe question is answered in the negative then at step 1404 the message“NOT PERMITTED” is sent back to the requesting terminal at step 1404.However, if it is answered in the affirmative then at step 1405 aquestion is asked as to whether the request is a CHANNEL PLAY request,indicating that a new communications link needs to be opened, or aCHANNEL CHANGE request, indicating that the link is open but that a newchannel is required.

If the answer is CHANGE then at step 1406 a CHANNEL CHANGE request issent in turn to media server 115. This request identifies the requestingdevice and the requested channel. At step 1407 a further question isasked as to whether a reply of “OK” is received. If this question isanswered in the negative then for some reason the media server cannotchange channel, probably because the communications link has beenbroken. Thus at this stage, or if the received request is a CHANNEL PLAYrequest, a request for a new URL is sent to media server 115. At step1409 the URL is received and at step 1410 it is sent to the requestingterminal in order that the terminal can open a TCP connection using theURL. The media controller takes no further part in the set-up of thecommunications link.

At this stage, or following an “OK” reply at step 1407, a question isasked at step 1411 as to whether another request has been received. Ifthis question is answered in the affirmative then control is returned tostep 1403 and the process is repeated. Eventually the question isanswered in the negative and the process is shut down at step 1412,usually with the switching off for some reason of content server 114.

Thus media controller acts as an intermediary between a terminal such asPDA 104 and media server 115, checking that a user is permitted to viewchannels before requesting media server 115 to fulfill the request.

FIG. 15

FIG. 15 shows media server 115, which is substantially similar tocontent server 114. It comprises two parallel central processing units(CPUs) 1501 and 1502 having a clock frequency of 3 GHz, a main memory1503 comprising 4 GB of dynamic RAM and local storage 1504 provided by a20 Gb-disk array. A CD-ROM disk drive 1505 allows instructions to beloaded onto local storage 1504 from a CD-ROM 1506. A first GigabitEthernet card 1507 facilitates intranet connection to RTSP servers 116,117 and 118. A second Gigabit Ethernet card 1508 provides a connectionto Internet 108.

FIG. 16

Media server 115 receives data streams from RTSP servers 116 to 118 thatare forwarded to the terminals on request. Media server 115 sets up aplurality of server channels 1601, 1602, 1603, 1604, 1605, 1606, 1607,1608 and 1609, each of which emulates an RTSP client and negotiates acommunications link in the usual way in order to receive audio-visualdata from the RTSP servers. Thus, for example, server channel 1601negotiates communications link 1610 with RTSP server 116 in order toreceive the two RTP streams and two RTCP streams that define a firsttelevision channel. This audio-visual data contains the same programmesthat are sent over the usual television and satellite networks, but itis encoded suitably for display on terminals, typically reducing theamount of data considerably. In this embodiment all the data is encodedusing the same encoder to allow easier switching of channels, but inother embodiments transcoding of the outgoing data could be used at theserver, thus allowing different encodings of the different incomingchannels.

For each terminal that has requested audio-visual data a user channel isdefined, such as user channel 1611 that communicates with PDA 104, userchannel 1612 that communicates with PC 106, user channel 1613 thatcommunicates with mobile telephone 103, and user channel 1614 thatcommunicates with mobile telephone 102. Each user channel emulates anRTSP server in order to communicate with the media player on itsrespective terminal.

Each user channel receives input from one server channel. Thus, forexample, user channel 1611 receives input from server channel 1601, userchannels 1612 and 1613 both receive input from server channel 1604, anduser channel 1614 receives input from server channel 1608. These inputsare the data received from the RTSP servers by the respective serverchannel. On first set-up of a user channel the input data is passed bythe user channel to the terminal without alteration. However, uponfulfillment of a channel change request the input is changed. Thus, whenPDA 104 requests a change of channel to the channel provided by serverchannel 1603 the media server 115 changes the input to user channel1611. The input represented by line 1615 is stopped and instead the datafrom server channel 1603 is input, as represented by line 1616.

However, once the channel has been changed the data can no longer besent unaltered to PDA 104. The display time within each RTP and RTCPpacket must be altered before being sent. Further, since the mediaplayer is expecting packet numbers to continue in sequence, the sequencenumbers of the packets must also be altered.

FIG. 17

The contents of main memory 1503 are illustrated in FIG. 17. Operatingsystem 1701 provides operating system instructions for common systemtasks and device abstraction. In this example a Windows® Serveroperating system is used, but another system providing similarfunctionality could be used. Channel manager 1702 receives requests frommedia controller 1303 and manages the server channels and user channels.It also includes a Quality of Service manager 1703 that monitors reportpackets received from terminals. Server channel objects 1704 defineserver channels 1601 to 1609 while user channel objects 1705 define userchannels 1611 to 1614. Data 1706 includes data used by operating system1701 and channel manager 1702.

FIG. 18

FIG. 18 details steps carried out by channel manager 1702. At step 1801it starts up, usually with the switching on of media server 115. At step1802 the first connected RTSP server is selected and at step 1803 thefirst channel that the server supplies is selected. At step 1804 aserver channel object is defined to receive the data and at step 1805 acommunications link is defined between the RTSP server and the serverchannel as an RTSP client. At step 1806 a question is asked as towhether there is another channel on the server, and if this question isanswered in the affirmative then control is returned to step 1803 andthe next channel is selected. If it is answered in the negative then atstep 1807 a further question is asked as to whether there is anotherconnected RTSP server, and if this question is answered in theaffirmative then control is returned to step 1802 and the next server isselected. Alternatively, if the question is answered in the negativethen all the necessary server channels have been defined.

Thus at step 1808 a request to play a channel is received from mediacontroller 1303 on content server 114. At step 1809 a question is askedas to whether the request is a CHANNEL PLAY or a CHANNEL CHANGE request.If the answer is PLAY then then at step 1810 a new user channel isdefined. Alternatively, if it is a CHANGE request then at step 1811 theexisting user channel for the requesting terminal is modified. Followingeither step, a question is asked at step 1812 as to whether anotherrequest has been received, and if this question is answered in theaffirmative then control is returned to step 1809 and the request isprocessed. Eventually the question is answered in the negative andchannel manager 1702 is shut down at step 1813, usually with theswitching off for some reason of media server 115.

FIG. 19

FIG. 19 details step 1810 at which a new user channel is defined. Atstep 1901 a question is asked as to whether a user channel alreadyexists for the requesting terminal. This may happen when acommunications link is broken and the terminal needs to renegotiate thelink with a new CHANNEL PLAY request, and so if the question is answeredin the affirmative then the existing user channel is deleted at step1902. At this stage, or if the question is answered in the negative,then a user channel is created at step 1903 and at step 1904 a URL iscreated that indicates either the IP address or a resolvable DNS addressof the media server, the TCP port that has been allocated to the userchannel, and a filename that will be recognized by the channel manageras indicating the defined user channel. This URL is then returned to themedia controller 1303 at step 1905 in order that it can be sent in turnto the requesting terminal. The URL is time limited for securitypurposes and thus at step 1906 a question is asked as to whether theterminal used the URL to open a TCP connection within thirty seconds andif this question is answered in the negative then at step 1907 the userchannel is deleted. Alternatively, the TCP connection is initiated intime and the user channel is set up at step 1908.

FIG. 20

FIG. 20 details step 1908 at which the user channel is set up, mainly byestablishing a communications link between the requesting terminal as anRTSP client and the defined user channel as an RTSP server. On theterminal side this is performed by the media player, such as mediaplayer 303, carrying out the steps detailed in FIG. 5. Thus at step 2001the channel manager 1702 receives a DESCRIBE request from the terminaland at step 1703 it sends a reply that includes an indication of thenumber and type of streams, such as one audio stream and one videostream. At step 2003 a SETUP request is received from the terminal forthe first stream, which includes the terminal port numbers that shouldbe used for the first RTP and RTCP data streams. A reply is sent at step2004 indicating the ports on the media server that the user channel isusing for these streams. At step 2005 a question is asked as to whetherthere is another stream, and if this question is answered in theaffirmative then control is returned to step 2003 and a SETUP request isreceived for that stream. Alternatively, the question is answered in thenegative and the TCP connection is closed at step 2006. At step 2007 thenecessary ports are opened and at step 2008 initial user channel offsetsof zero are stored in the user channel object. This means that thedisplay times contained in the data packets will be offset by zero, ieleft unaltered, as will the sequence numbers of the packets.

FIG. 21

Step 1811 at which a user channel is altered following a CHANNEL CHANGErequest is detailed in FIG. 21. At step 2101 a question is asked as towhether the user channel is still open, since it is possible that thechannel manager may have closed it due to a long period of inactivityfollowing a broken communications links. If this question is answered inthe affirmative then at step 2102 the message “NOT OK” is sent back tomedia controller 1303 and step 1811 is exited. However, under normalcircumstances the user channel is open and sending data to the terminal.Thus at step 2103 the channel offsets, ie the display time offsets andthe sequence number offsets, are changed in the user channel object andat step 2104 the input to the user channel is changed to the serverchannel that has the requested television channel. The message “OK” isthen sent to media controller 1303 at step 2105.

FIG. 22

FIG. 22 illustrates the effect of changing the display time offsets atstep 2103, which will be described further with respect to FIG. 23. Thegraph plots server time against display time for four RTP streams, afirst video stream 2201 and a first audio stream 2202, which are thestreams that are the current input into the user channel, and a secondvideo stream 2202 and a second audio stream 2204, which carry thetelevision channel that the user wishes to change to.

For each of the streams, the display time corresponding to an servertime T shown by line 2205 can be calculated from the RTCP packetscorresponding to each stream. This is done in this example byextrapolating from the last four RTCP packets for each stream. Thedifference in display times D1, shown by arrow 2206, between the twovideo streams and the difference in display times D2, shown by arrow2207, between the two video streams can then be calculated. D1 is addedto the current video offset and D2 is added to the current audio offset,both of which are zero at first set-up of the user channel, to produce avideo offset 2208 and an audio offset 2209. Every packet that is sent bythe user channel is altered by adding the video offset to the displaytime in the RTP and RTCP packets for the video stream, and by adding theaudio offset to the display time in the RTP and RTCP packets for theaudio stream.

Thus the second video stream is offset as shown by line 2210 and thesecond audio stream is offset as shown by line 2211. The playerreceiving the packets will thus display them at a modified server time.The gradients of the streams are different, leading to a slight speedingup or slowing down at first, but this will be corrected once two orthree RTCP packets have been received by the player. The user will notnotice this slight change in speed as long as the audio and videostreams are synchronized.

In an embodiment where the client is not able to change the gradientonce it has been set, all the streams would have to have the samegradient. In practice, it is likely that servers 116, 117 and 118 willuse preset gradients rather than randomly-generated ones and thus allthe streams would have the same gradient.

The internal clocks on servers 116, 117 and 118 may not be synchronized,and if this is the case then the server time in data packets originatingfrom different servers will be different. The result of this is thatwhen switching between streams from different servers there may beeither a jump backwards or a delay in viewing of the data, which will beequal to the difference between the servers' internal clocks. A jumpbackwards in time would probably not be noticed unless the server timewere extremely inaccurate, but a delay would be noticed and nottolerated by a user. Thus the offsets may need to be augmented to takeaccount of this fact, and this is described further with reference toFIG. 23 a.

FIG. 23

FIG. 23 details step 2103 at which the channel offsets are changed asillustrated in FIG. 22. At step 2301 an server time T is defined and atstep 2302 a first type of stream, video or audio, is selected. At step2303 the display time of the stream that is no longer required iscalculated for the server time T, and at step 2304 the same iscalculated for the new stream. At step 2305 the difference D between thedisplay times is calculated and at step 2306 this difference is added tothe current display time offset for the selected stream type. At step2307 the packet sequence number difference is calculated by subtractingthe sequence number of the next packet in the old stream from thesequence number of the next packet in the new stream and adding it tothe current sequence number offset for the selected stream type at step2308.

At step 2309 a question is asked as to whether there is another type ofstream and if this question is answered in the affirmative then controlis returned to step 2302 and the next type of stream is selected.Alternatively, the question is answered in the negative and at step 2110both of the display time offsets are augmented.

FIG. 23a

The display time offsets for both the video and audio streams may needto be augmented to take account of different server times. If the oldand new channels come from the same server, or servers havingsynchronized internal clocks, this step will result in no change to theoffsets. However, if the channels come from different servers havingnon-synchronized clocks this step will ensure a smooth transitionbetween channels.

At step 2311 the video stream in the new channel is selected and at step2312 the calculated display time offset for the video stream is added toit. At step 2313 the last packet that was sent in the old video datastream is selected and at step 2314 the difference between the displaytime in this last packet and the offset display time in the new packetis calculated. Between two channels with no difference in server time,this difference will be zero, but between channels coming fromdifferent, non-synchronized servers, this difference will not be zero.

Thus at step 2315 the difference is converted to a server time using thefunction for the old video stream, and at step 2316 this server time isconverted to display time using the function for the new video stream.The result of this is added to the video display time offset at step2317. Similarly, at step 2318 the same server time is converted todisplay time using the function for the new audio stream. The result ofthis is added to the audio display time offset at step 2319.

This means that for both audio and video, the first data packet in thenew stream has the same display time as the last packet in the oldstream and thus any difference between server clocks is allowed for.

FIG. 24

FIG. 24 shows a further illustration of the changing of display timeoffsets (assuming that the servers are synchronized and no augmentationis necessary). The input to the user channel has changed to the secondvideo stream 2203 and 2204, but modified data packets are being sentwith an offset display time as shown by lines 2210 and 2211. AnotherCHANNEL CHANGE request has been received and so the input to the userchannel will change to third video stream 2401 and third audio stream2402. Thus at time T, shown by line 2403, the display times for themodified second video and audio streams 2210 and 2211 and the thirdvideo and audio streams 2401 and 2402 are calculated. The difference indisplay times D1, shown by arrow 2404, between the two video streams andthe difference in display times D2, shown by arrow 2405, between the twoaudio streams can then be calculated. D1 is added to the current videooffset and D2 is added to the current audio offset to produce a videooffset 2406 and an audio offset 2407. Thus the third video stream isoffset as shown by line 2408 and the second audio stream is offset asshown by line 2409.

FIG. 25

FIG. 25 is a block diagram showing the streams that pass along thecommunication links. Media server 115 communicates with a terminal, forexample PDA 104, using user channel 1611 which is taking input from, forexample, server channel 1608. RTP data packets 2501 are received byserver channel 1608 from RTSP server 116 and passed to user channel 611,which applies the display time and sequence number offsets and sendsthem to PDA 104 along communications link 2502 as modified RTP datapackets 2503. RTCP control packets 2504 are received by server channel1608 and passed to user channel 611, which applies the offsets and sendsthem to PDA 104 along communications link 2502 as modified RTCP datapackets 2505.

Server channel 1608 creates RTCP report packets 2506 and sends them tothe RTSP server 116. PDA 104 also creates report packets 2507 and sendsthem via communications link 2502 to user channel 1611, which forwardsthem to channel manager 1702.

The bandwidth available to communications links over internet 108fluctuates constantly and thus at any one time the packet loss may bevery high or very low. RTCP report packets include an indication ofpacket loss, expressed as a fraction known as a loss fraction, andQuality of Service (QoS) manager 1703, which is part of channel manager1702, uses this information to alter the service supplied to PDA 104. Ifthe packet loss is high then there will be many gaps in the audio-visualdata being viewed by the user, particularly if keyframes are being lost.The user will probably prefer to view a lower quality version of thetelevision channel than continue with gaps in the viewing. Thus whenpacket loss is very high a downwards adjustment in quality can be made.

One way in which this can be performed is by having more than oneencoding of each television channel, with each encoding being of adifferent quality, and thus having a different amount of data. Thus, forexample, referring back to FIG. 16, server channel 1601 receives datarepresenting a first television channel encoded at 30 kbits/second,server channel 1602 receives the same television channel but encoded at50 kbits/second, and server channel 1603 receives the same televisionchannel but encoded at 100 kbits/second. Thus each server channelreceives audio-visual data representing the same information, ie thesame television channel, with each audio-visual data being encoded at adifferent data rate. Because the method of channel switching describedherein provides virtually seamless switching, it is possible to stepdown, ie switch from a higher encoding to a lower encoding,transparently to the user. Thus first audio-visual data containing firstinformation encoded at a first data rate and second audio-visual datacontaining first information encoded at a second data rate that is lessthan the first data rate are received. The first audio-visual data isprovided to a network-connected terminal, before a condition to theeffect that the amount of data being sent to the terminal exceeds theavailable bandwidth is identified. Provision of the first audio-visualdata to the terminal is stopped and the second audio-visual data isprovided instead.

However, because of the nature of the Internet 108 it is extremelylikely that temporary high packet loss will occur occasionally, leadingto a step down in quality. It is therefore necessary to step up, ieswitch from a lower encoding to a higher encoding, should packet loss bevery low. Thus a condition to the effect that the amount of data beingsent to the terminal is less than the available bandwidth is identified,provision of the second audio-visual data to the terminal is stopped,and the first audio-visual data is provided again to the terminal.

However, low packet loss only indicates that the current amount of datais acceptable. For example, if data is being sent at 50 kbits/second andthere is 70 kbits/second of available bandwidth then packet loss will below, but attempting to increase the data to 100 kbits/second will resultin extreme packet loss and, contrary to intentions, a degradation ofquality for the user. Thus instead of stepping up immediately, QoSmanager 1703 sends probe packets 2508, which contain dummy data, to thesame ports on the receiving terminal as the RTCP packets. They arediscarded by the media player because they are not RTCP packets. Byincreasing the number of probe packets 2508 sent until the total amountof data being transmitted is equal to the amount required by the higherencoding, QoS manager 1703 can ensure that the bandwidth required isavailable before stepping up. Thus the difference between the data ratesof the first and second audio-visual data is calculated, a condition tothe effect that the difference is larger than a specified threshold isidentified, and additional data packets are sent to the terminal todetermine whether the available bandwidth is large enough to provide thefirst audio-visual data to the terminal.

FIG. 26

FIG. 26 shows the steps carried out by QoS manager 1703. At step 2601 itstarts up when channel manager 1702 starts and at step 2602 a reportpacket is received from a user channel. At step 2603 the correspondinguser channel object is selected and at step 2604 the data in the objectis examined to determine whether more than two report packets have beenreceived since the last logged event. An event may be a step down inquality, a step up, or a failed attempt to step up. The loss fraction inthe first two report packets after an event tends to be distorted by theevent and so they are not considered. Thus if the question asked at step2604 is answered in the affirmative then at step 2605 the loss fractioncontained in the report packet is stored in the user channel object.

Loss fractions are examined in samples of four and so at step 2606 aquestion is asked as to whether there are four loss fractions stored inthe user channel object, and if this question is answered in theaffirmative then a further question is asked as to whether two of theloss fractions are greater than 100/256. If this question is answered inthe affirmative then at step 2608 a failed sample is logged, whereas ifit is answered in the negative a passed sample is logged at step 2609.In either case, the loss fractions are cleared from the object in orderto start a new sample.

The quality of the channel is then changed if appropriate at step 2611and at step 2612 a question is asked as to whether there is anotherreport packet. This step also follows an answer in the negative to thequestion asked at step 2604 or step 2606. If it is answered in theaffirmative then control is returned to step 2603. Eventually it isanswered in the negative and the manager terminates at step 2613,usually with the switching off for some reason of media server 115.

FIG. 27

FIG. 27 details step 2611 at which the quality of the channel is changedif appropriate. At step 2701 a question is asked as to whether a periodof time, in this example at least thirty seconds, has lapsed since thelast event, again because events can distort the data. If this questionis answered in the affirmative then at step 2702 a question is asked asto whether there have been three consecutive passed samples. If thisquestion is answered in the affirmative then a step up in quality may beappropriate and this is carried out at step 2703. Alternatively, if thequestion asked at step 2702 if this question is answered in the negativethen a further question is asked at step 2704 as to whether the lastsample failed. If this question is answered in the affirmative then astep down in quality may be appropriate and this is carried out at step2705. If this question or the question asked at step 2701 is answered inthe negative then no change in quality is made.

FIG. 28

FIG. 28 details step 2703 at which a step-up in quality may be carriedout. If there have been three consecutive passed samples then clearlythe current amount of data being sent on the communications link isgetting through and there may be bandwidth for more data, and thus ahigher quality of picture, both audio and video, could be shown. If thejump in the amount of data between encodings is not very large then anattempt at stepping up could be made, since the likelihood of steppingfar beyond the packet rate that can be supported is low, and even if thenew rate cannot be supported a step back down can be made without havingdegraded the user experience significantly. Alternatively, probe packetsare sent to check the available bandwidth.

Thus at step 2801 a question is asked as to whether probe packets arecurrently being sent. If this question is answered in the negative thenat step 2802 a question is asked as to whether a higher-quality encodingof the television channel is available. If this question is answered inthe negative then there is no need to pursue the question of stepping upand so step 2703 is terminated. However, if it is answered in theaffirmative then at step 2803 a further question is asked as to whetherthe difference in bandwidth between the current encoding and thehigher-quality encoding is greater than 32 kbits/second. If thisquestion is answered in the affirmative then the sending of probepackets 2708 is commenced at step 2804, as will be described furtherwith reference to FIG. 30. Alternatively, if the question is answered inthe negative then the difference between encodings is small enough toattempt a speculative step-up and thus at step 2805 a CHANNEL CHANGErequest for the higher-quality encoding is sent to channel manager 1702,which processes it in the same way as any other CHANNEL CHANGE request.

FIG. 29

FIG. 29 details step 2705 at which a step down in quality may be made.At step 2901 a question is asked as to whether probe packets arecurrently being sent. If this question is answered in the affirmativethen the failed sample means that a step up will not be possible andthus at step 2902 the probe packets are stopped by terminating theprocess described with reference to FIG. 30. However, if it is answeredin the negative then at step 2903 a further question is asked as towhether a lower-quality encoding of the television channel is available.If this question is answered in the negative then nothing can be doneabout the packet loss and step 2705 is terminated. However, if it isanswered in the affirmative then at step 2904 a CHANNEL CHANGE requestfor the lower-quality encoding is sent to channel manager 1702, whichprocesses it in the same way as any other CHANNEL CHANGE request.

Thus a failed sample results in the cessation of probe packets if theyare being sent, or in a step down in quality if they are not being sent

FIG. 30

FIG. 30 illustrates the probing mechanism, which is a separate thread onQoS manager, and which is started at step 2804 after three consecutivepassed samples when the difference in bandwidth between versions isgreater than 32 kbits/second. Thus at step 3001 the mechanism isstarted. At step 3002 a step size is calculated to be one tenth of thedifference in bandwidth between the versions, and at step 3003 a probinginterval is calculated as eight times the average time between receiptof report packets. At step 3004 the probing starts by sending dummy datathat increases the bandwidth used by a number of step sizes, called thelevel. Probing starts at level one.

The process then waits at step 3005 for the calculated probing intervalbefore asking the question at step 3006 as to whether level ten ofprobing has been reached yet. If this question is answered in thenegative then the level is increased by one at step 3007 and control isreturned to step 3005 where the process again waits. Eventually thequestion asked at step 3006 is answered in the affirmative, to theeffect that level ten has been reached. This means that the amount ofbandwidth required by the probe data plus the current version is equalto the amount of bandwidth required by the higher quality version. Theprocess waits at step 3008 for twice the probing interval to ensure thatthis bandwidth is sustainable before sending a CHANNEL CHANGE requestfor the higher-quality encoding to channel manager 1702 at step 3009,which processes it in the same way as any other CHANNEL CHANGE request.The process then terminates at step 3010.

Alternatively, should a failed sample be logged at any time then the QoSmanager will terminate the probing process at step 2902.

FIG. 31

FIG. 31 illustrates how the QoS manager 1703 responds to changes inavailable bandwidth, indicated by the packet loss fraction, by steppingup or stepping down the quality of the television channel supplied to aterminal. The line 3101 plots available bandwidth in kbits/secondagainst time. Four encodings of the same channel are available and areshown on the bandwidth axis according to how much bandwidth theyrequire. The first encoding is at 30 kbits/second, the second at 50kbits/second, the third at 100 kbits/second and the fourth at 160kbits/second.

At first, as shown by line 3102, the third encoding at 100 kbits/secondis transmitted. The available bandwidth 3101 starts to fall, and indeedat point 3103 falls below 100 kbits/second but not for long enough toproduce a failed sample. However, at time 3104 a failed sample leads toa step-down in quality to the second encoding at 50 kbits/second. As thebandwidth 3101 continues to fall a further step-down is made at time3105 to the lowest-quality encoding at 30 kbits/second.

The bandwidth starts to improve and three consecutive passed samples attime 3106 lead to a step-up in quality to 50 kbits/second. Since thedifference between the encodings is only 20 kbits/second a speculativestep-up rather than probe data is possible. At time 3107 probing starts,and thus while the amount of RTP/RTCP data being sent is still at 50kbits/second, as shown by line 3108, the total data sent over thecommunications link is higher, as shown by line 3109. However, theavailable bandwidth 3101 is not high enough and thus at time 3110 afailed sample means that the probing is stopped. Probing starts again attime 3111 and this time is successful, resulting in a step up to thethird encoding at 100 kbits/second at time 3112. The bandwidth continuesto rise and thus at time 3113 probing starts again in order to test thepossibility of a step up to the highest encoding.

1. A method of providing audio-visual data over a network, comprisingthe steps of: receiving first audio-visual data containing firstinformation encoded at a first data rate, and second audio-visual datacontaining first information encoded at a second data rate that ishigher than said first data rate; providing said first audio-visual datato a network-connected terminal; identifying a condition to the effectthat the amount of data being sent to said terminal is less than theavailable bandwidth; calculating the difference between the data ratesof said first and second audio-visual data; sending additional datapackets to said terminal to determine whether the available bandwidth islarge enough to provide said first audio-visual data to said terminal;and if it is determined that the available bandwidth is large enough,stopping provision of said first audio-visual data to said terminal andproviding said second audio-visual data to said terminal.
 2. A methodaccording to claim 1, wherein the step of sending additional datapackets to said terminal comprises the steps of: defining a first amountof data and sending additional data packets, the sum of the data in eachbeing equal to said first amount of data; and gradually increasing theamount of data sent until the additional amount of data being sent isequal to said difference.
 3. A method according to claim 2, wherein saidstep of gradually increasing the amount of data sent comprisesincreasing the amount of data in each additional data packet.
 4. Amethod according to claim 2, wherein said step of gradually increasingthe amount of data sent comprises increasing the number of additionaldata packets sent.
 5. A method according to claim 1, further comprisingthe steps of: identifying a condition to the effect that said differenceis smaller than a specified threshold, and stopping provision of saidfirst audio-visual data and providing second audio-visual data to saidterminal without sending additional data packets.
 6. A method accordingto claim 1, further comprising the steps of, after the sending ofadditional data packets has commenced: identifying a condition to theeffect that the amount of data being sent to said terminal exceeds theavailable bandwidth; and stopping said additional data packets.
 7. Amethod according to claim 1, wherein said step of identifying acondition to the effect that the amount of data being sent to theterminal is less than the available bandwidth comprises the steps of:monitoring a measurement of packet loss; and identifying when saidmeasurement of packet loss falls below a second specified threshold. 8.A method according to claim 7, wherein said measurement of packet lossis a sample of four loss fractions, wherein each of said loss fractionsis contained within a report packet received from said network-connectedterminal and represents a percentage of recent packets that have notbeen received by said network-connected terminal.
 9. A method accordingto claim 8, wherein said measurement of packet loss falls below saidsecond specified threshold when three consecutive samples are passed,wherein a passed sample contains three loss fractions that do not exceed100/256.
 10. A method according to claim 1, further comprising the stepsof, subsequent to said second audio-visual data being provided to saidnetwork-connected terminal: identifying a condition to the effect thatthe amount of data being sent to said terminal is greater than theavailable bandwidth; stopping provision of said second audio-visual datato said terminal; and providing said first audio-visual data to saidterminal.
 11. A method according to claim 10, wherein said step ofidentifying a condition to the effect that the amount of data being sentto the terminal is greater than the available bandwidth comprises thesteps of: monitoring a measurement of packet loss; and identifying whensaid measurement of packet loss exceeds a first specified threshold. 12.A method according to claim 11, wherein said measurement of packet lossis a sample of four loss fractions, wherein each of said loss fractionsis contained within a report packet received from said network-connectedterminal and represents a percentage of recent packets that have notbeen received by said network-connected terminal.
 13. A method accordingto claim 12, wherein said first specified threshold is exceeded when twoout of said four loss fractions exceed 100/256.
 14. A method accordingto claim 1, wherein each of said audio-visual data comprises a pluralityof packets, and whenever the audio-visual data provided to said terminalis changed from a previous audio-visual data to a new audio-visual data,at least one characteristic of the packets in said new audio-visual datais modified to match the packets in said previous audio-visual data. 15.A method according to claim 14, wherein each of said packets includes adisplay time, and said step of modifying a characteristic of packet insaid new audio-visual data comprises the steps of: calculating a displaytime offset; and applying said display time offset to the display timein each packet in said new audio-visual data.
 16. A method according toclaim 15, wherein said step of modifying a characteristic of packets insaid new audio-visual data comprises applying a sequence number offsetto the sequence number of each packet in said new audio-visual data. 17.Apparatus for providing data over a network, comprising a processor,memory, a first network connection and a second network connection,wherein said processor is configured to: receive, via said first networkconnection, first audio-visual data containing first information encodedat a first data rate, and second audio-visual data containing firstinformation encoded at a second data rate that is higher than said firstdata rate; provide, via said second network connection, said firstaudio-visual data to a network-connected terminal; identify saidcondition to the effect that the amount of data being sent to theterminal is less than the available bandwidth; calculate the differencebetween the data rates of said first and second audio-visual data; sendadditional data packets to said terminal to determine whether theavailable bandwidth is large enough to provide said first audio-visualdata to said terminal; and if it is determined that the availablebandwidth is large enough, stop provision of said first audio-visualdata to said terminal and provide said second audio-visual data to saidterminal.
 18. Apparatus according to claim 17, wherein said processor isconfigured to send additional data packets to said terminal by: defininga first amount of data and sending additional data packets, the sum ofthe data in each being equal to said first amount of data; and graduallyincreasing the amount of data sent until the additional amount of databeing sent is equal to said difference.
 19. Apparatus according to claim18, wherein said said processor is configured to gradually increase theamount of data sent by increasing the amount of data in each additionaldata packet.
 20. Apparatus according to claim 18, wherein said saidprocessor is configured to gradually increase the amount of data sent byincreasing the number of additional data packets sent.
 21. Apparatusaccording to claim 17, said processor is further configured to: identifya condition to the effect that said difference is smaller than aspecified threshold, and stop provision of said first audio-visual dataand provide second audio-visual data to said terminal without sendingadditional data packets.
 22. A method according to claim 17, whereinsaid processor is further configured to, after the sending of additionaldata packets has commenced: identify a condition to the effect that theamount of data being sent to said terminal exceeds the availablebandwidth; and stop sending said additional data packets.
 23. Apparatusaccording to claim 17, wherein said processor is configured to identifysaid condition to the effect that the amount of data being sent to theterminal is less than the available bandwidth by: monitoring ameasurement of packet loss; and identifying when said measurement ofpacket loss falls below a second specified threshold.
 24. Apparatusaccording to claim 23, wherein said measurement of packet loss is asample of four loss fractions, wherein each of said loss fractions iscontained within a report packet received from said network-connectedterminal and represents a percentage of recent packets that have notbeen received by said network-connected terminal.
 25. Apparatusaccording to claim 24, wherein said measurement of packet loss fallsbelow said second specified threshold when three consecutive samples arepassed, wherein a passed sample contains three loss fractions that donot exceed 100/256.
 26. Apparatus according to claim 17, wherein saidprocessor is further configured to, subsequent to said secondaudio-visual data being provided to said network-connected terminal:identify a condition to the effect that the amount of data being sent tosaid terminal exceeds than the available bandwidth; stop provision ofsaid second audio-visual data to said terminal; and provide said firstaudio-visual data to said terminal.
 27. Apparatus according to claim 26,wherein said processor is configured to identify said condition to theeffect that the amount of data being sent to the terminal exceeds theavailable bandwidth by: monitoring a measurement of packet loss; andidentifying when said measurement of packet loss exceeds a firstspecified threshold.
 28. Apparatus according to claim 27, wherein saidmeasurement of packet loss is a sample of four loss fractions, whereineach of said loss fractions is contained within a report packet receivedfrom said network-connected terminal and represents a percentage ofrecent packets that have not been received by said network-connectedterminal.
 29. Apparatus according to claim 30, wherein said firstspecified threshold is exceeded when two out of said four loss fractionsexceed 100/256.
 30. Apparatus according to claim 17, wherein each ofsaid audio-visual data comprises a plurality of packets, and wheneverthe audio-visual data provided to said terminal is changed from aprevious audio-visual data to a new audio-visual data, said processor isconfigured to modify at least one characteristic of the packets in saidnew audio-visual data to match the packets in said previous audio-visualdata.
 31. Apparatus according to claim 30, wherein each of said packetsincludes a display time, and said processor is configured to modify saidcharacteristic of said packets by: calculating a display time offset;and applying said display time offset to the display time in each packetin said new audio-visual data.
 32. Apparatus according to claim 30,wherein said processor is configured to modify said characteristic ofsaid packets by applying a sequence number offset to the sequence numberof each packet in said new audio-visual data.